1. Field of the Invention
This invention relates to a system for feedback control of the air/fuel mixture ratio in an internal combustion engine. The system includes an air/fuel ratio detector having an oxygen-sensitive element of an oxygen concentration cell type operated with the supply of a DC current to establish a reference oxygen partial pressure in the element and is further provided with an electric heater to ensure the proper functioning of the element. More particularly, the invention relates to a sub-system to control a heating current to the heater of the oxygen-sensitive element preferably together with controlling the intensity of the current for establishing the reference oxygen partial pressure.
2. Background of the Invention
In recent internal combustion engines and particularly in automotive engines, it has become popular to control the air/fuel mixture ratio precisely to a predetermined optimal value by performing feedback control with the dual object of improving the efficiencies of the engines and reducing the emission of noxious or harmful substances contained in exhaust gases.
For example, in an automotive engine system including a catalytic converter which is provided in the exhaust passage and which contains a so-called three-way catalyst that can catalyze both the reduction of nitrogen oxides and oxidation of carbon monoxide and unburned hydrocarbons, it is desirable to control the air/fuel mixture ratio at a stoichiometric ratio because the catalyst exhibits the highest conversion efficiencies in an exhaust gas produced by the combustion of a stoichiometric air-fuel mixture, and also because the employment of a stoichiometric mixing ratio is favorable for realizing high mechanical and thermal efficiencies in the engine. It is already known to practice feedback control of the air/fuel ratio in such an engine system by using a sort of oxygen sensor, which is installed in the exhaust passage upstream of the catalytic converter together with a device that provides an electrical feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine. Based on this feedback signal, a control circuit commands a fuel-supplying apparatus such as electronically controlled fuel injection valves to control the rate of fuel feed to the engine so as to nullify or minimize deviations of the actual air/fuel ratio from the intended stoichiometric ratio.
Usually the above mentioned oxygen sensor is of an oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte, such as zirconia stabilized with calcia, and conventionally the sensor is constituted of a solid electrolyte layer in the shape of a tube closed at one end, a measurement electrode layer porously formed on the outer side of the solid electrolyte tube and a reference electrode layer formed on the inner side of the tube. When there is a difference in oxygen partial pressure between the reference electrode side and measurement electrode side of the solid electrolyte tube, this sensor generates an electromotive force between the two electrode layers. As an air/fuel ratio detector for the above mentioned purpose, the measurement electrode is exposed to an engine exhaust gas while the reference electrode on the inside is exposed to atmospheric air utilized as the source of a reference oxygen partial pressure. In this state the magnitude of the electromotive force generated by this sensor exhibits a great and sharp change between a maximally high level and a very low level each time when the air/fuel ratio of a mixture supplied to the engine changes across the stoichiometric ratio. Accordingly it is possible to produce a fuel feed rate control signal based on the result of a comparison of the output of the oxygen sensor with a reference voltage which has been set at the middle of the high and low levels of the sensor output.
However, this type of oxygen sensor has disadvantages such as significant temperature dependence of its output characteristics, necessity of using a reference gas such as air, difficulty in reducing the size and insufficiency of mechanical strength.
To eliminate such disadvantages of the conventional oxygen sensor, U.S. Pat. No. 4,207,159 discloses an advanced device comprising an oxygen-sensitive element in which an oxygen concentration cell is constituted of a flat and microscopically porous layer of solid electrolyte, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side on a base plate or substrate such that the reference electrode layer is sandwiched between the substrate and the solid electrolyte layer and macroscopically shielded from the environmental atmosphere. Each of the three layers on the substrate can be formed as a thin, film-like layer. This device does not use any reference gas. Instead, a DC power supply means is connected to the oxygen-sensitive element so as to force a constant DC current (e.g. of a current intensity of about 10 .mu.A) to flow through the solid electrolyte layer between the two electrode layers to thereby cause migration of oxygen ions through the solid electrolyte layer in a selected direction and, as a consequence, establish a reference oxygen partial pressure at the interface between the solid electrolyte layer and the reference electrode layer, while the measurement electrode layer is made to contact an engine exhaust gas. Where the current is forced to flow through the solid electrolyte layer from the reference electrode layer toward the measurement electrode layer, there occur ionization of oxygen contained in the exhaust gas at the measurement electrode and a migration of negatively charged oxygen ions through the solid electrolyte layer toward the reference electrode. The rate of supply of oxygen in the form of ions to the reference electrode is primarily determined by the intensity of the current. The oxygen ions arriving at the reference electrode layer are deprived of electrons and turn into oxygen molecules to result in an accumulation of gaseous oxygen on the reference electrode side of the concentration cell. However, a portion of the accumulated oxygen molecules diffuse outwardly through the microscopical gas passages in the solid electrolyte layer. Therefore, it is possible to maintain a constant and relatively high oxygen partial pressure which can serve as a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer by the employment of an appropriate current intensity with due consideration of the microscopical structure and activity of the solid electrolyte layer. Generated between the reference and measurement electrode layers of this oxygen-sensitive element is an electromotive force, the magnitude of which is related to the composition of the exhaust gas and the air/fuel ratio of a mixture from which the exhaust gas is produced. Also, it is possible to operate this oxygen-sensitive element by forcing a current to flow therein from the measurement electrode layer toward the reference electrode layer. In this case a constant and relatively low oxygen partial pressure can be maintained at the interface between the reference electrode layer and the solid electrolyte layer.
To supply a DC current of an accurately constant intensity, use is made of a constant current supply circuit including conventional electronic control means.
Devices such as that described in U.S. Pat. No. 4,207,159 have the advantages that it is unnecessary to use any reference gas, it can be made in a very small size and has good resistance to mechanical shocks and vibrations.
In practical applications it becomes necessary to provide this device (also conventional oxygen sensors of the solid electrolyte concentration cell type) with an electric heater because the activity of the solid electrolyte layer in the device becomes unsatisfactorily low while the temperature of the oxygen-sensitive element is relatively low, e.g. is below about 400.degree. C. Therefore, the oxygen-sensitive element installed in an engine exhaust system becomes ineffective as an air/fuel ratio detector while the engine discharges a relatively low temperature exhaust gas if the element should be heated solely by the heat of the exhaust gas. The electric heater is usually attached to, or embedded in, the substrate of the oxygen-sensitive element.
In automotive engines provided with an air/fuel ratio feedback control system as mentioned hereinbefore utilizing an air/fuel ratio detector according to U.S. Pat. No. 4,207,159, it is usual to temporarily release the air/fuel ratio from feedback control while the engine is operated under certain high-load conditions as typified by a full-throttle or nearly full-throttle accelerating condition in order to supply a fuel-enriched mixture to the engine and attain good accelerating performance. Under such conditions, there occurs considerable lowering of the oxygen concentration in the exhaust gas and a considerable rise in the exhaust gas temperature. Then, there occurs an unfavorable phenomenon in that the magnitude of a reference oxygen partial pressure on the reference electrode side of the oxygen-sensitive element decreases considerably even though the supply of a current of a constant intensity to this element is continued. This phenomenon occurs because, although the migration of oxygen ions through the solid electrolyte layer toward the reference electrode layer by the effect of the constant current flowing in the solid electrolyte layer continues, the outward diffusion of gaseous oxygen from the reference electrode through the solid electrolyte layer into the exhaust gas of a lowered oxygen concentration augments. Such a deviation of the reference oxygen partial pressure from an initially intended value offers little problem so long as the feedback control of air/fuel ratio is discontinued. However, when the feedback control is resumed the lowered reference oxygen partial pressure does not instantly revert to the initially intended value but gradually rises to this value, meaning that the recovery of the initially intended level of reference oxygen partial pressure takes a relatively long period of time compared with the frequencies of the feedback signal produced by the air/fuel ratio detector and the control signal supplied to the fuel system. Therefore, during this time period it becomes impossible to accurately control the air/fuel ratio to a predetermined ratio such as a stoichiometric ratio.